How to tell g++ compiler where to search for include files?

Question

In a "working directory" I have a lot of *.cpp and *.h files that #include each other and files from subdirectories.

For example:

#include "first.h" #include "second.h" #include "dir1/third.h" #include "dir2/fourth.h" 

In my own directory (that is different from the "working" directory) I would like to create a new *.cpp and *.h file that includes one of the files from the "working" directory. For example:

#include "/root/workingdirectory/first.h" 

However, it does not work. Because "first.h" might include "second.h" and "second.h" is not located in my directory. Is there a way to tell the compiler that it needs to search included files not in the current but in the working directory: /root/workingdirectory/?

To make it even more complex, dir1 and dir2 are not located in my working directory. They are located in /root/workingdirectory2/. So, my second question is if it is possible to resolve this problem by letting compiler know that subdirectories are located somewhere else?

I also need to add, that I do not use any environment for development and compile from the command line (using g++).

Answer 1

As you've already been told, it's useful to read the manual - specifically this chapter - and even more specifically right here.

Specifically, you want

g++ -I/root/workingdirectory -I/root/workingdirectory2 

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Note also the documentation on #include directive syntax, described here as:

2.1 Include Syntax

Both user and system header files are included using the preprocessing directive #include. It has two variants:

#include <file> 

This variant is used for system header files. It searches for a file named file in a standard list of system directories. You can prepend directories to this list with the -I option (see Invocation).

#include "file" 

This variant is used for header files of your own program. It searches for a file named file first in the directory containing the current file, then in the quote directories and then the same directories used for <file>. You can prepend directories to the list of quote directories with the -iquote option. The argument of #include, whether delimited with quote marks or angle brackets, behaves like a string constant in that comments are not recognized, and macro names are not expanded. Thus,#include <x/*y> specifies inclusion of a system header file named x/*y.

However, if backslashes occur within file, they are considered ordinary text characters, not escape characters. None of the character escape sequences appropriate to string constants in C are processed. Thus, #include "x\n\\y" specifies a filename containing three backslashes. (Some systems interpret \ as a pathname separator. All of these also interpret / the same way. It is most portable to use only /.)

It is an error if there is anything (other than comments) on the line after the file name.

So for example

#include "first.h" 

will start looking in the same directory as the .cpp file containing this directive (or take a relative path as relative to this directory).

If you want to use the include path (specified by -I) you should use

#include <dir1/third.h> 

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Usual practice is to use the #include "local.h" form for headers inside a library/package/module (however you've chosen to organize that), and the #include <external.h> form for headers from external/3rd-party or system libraries.

Answer 2

Read The Fine Manual

It's there for everyone to read. You even have a choice of what to use (I'd go with the first):

-Idir 

Add the directory dir to the head of the list of directories to be searched for header files. This can be used to override a system header file, substituting your own version, since these directories are searched before the system header file directories. However, you should not use this option to add directories that contain vendor-supplied system header files (use -isystem for that). If you use more than one -I option, the directories are scanned in left-to-right order; the standard system directories come after.

If a standard system include directory, or a directory specified with -isystem, is also specified with -I, the -I option is ignored. The directory is still searched but as a system directory at its normal position in the system include chain. This is to ensure that GCC's procedure to fix buggy system headers and the ordering for the include_next directive are not inadvertently changed. If you really need to change the search order for system directories, use the -nostdinc and/or -isystem options.

-iquotedir 

Add the directory dir to the head of the list of directories to be searched for header files only for the case of #include "file"; they are not searched for #include <file>, otherwise just like -I.